1. Field of the Invention
This invention relates to magnetic recording disks widely used as storage media in computers, such as hard disks. This invention also relates to manufacture of such magnetic recording disks.
2. Description of the Related Art
Magnetic recording disks such as hard disks and flexible disks are widely used as storage media in computers. A magnetic recording disks have a basic structure where a magnetic recording layer is provided on a disk-shaped substrate.
Manufacture of a magnetic recording disk is described as follows, taking a hard disk as an example. Conventionally, a substrate made of aluminum is employed in manufacturing a hard disk. A NiP (nickel phosphide) film is deposited on the aluminum-made substrate. On the NiP film, such a film as CoCr film is deposited as an underlying layer. On the underlying layer, such a film as CoCrTa film is deposited as a magnetic film for the magnetic recording layer. On the magnetic recording layer, a carbon film having a structure similar to diamond, which is called “diamond-like-carbon (DLC) film”, is deposited as a protection layer called “overcoat”.
In manufacture of magnetic recording disks, several limitations are foreseen from the point of view increasing recording density. This point is described as follows. Recent recording density in magnetic recording disks has been soaring remarkably. Currently it is reaching 35 gigabit/inch2, supposedly 100 gigabit/inch2 in the future. For higher recording density, it is necessary to make magnetic domains shorter and track width narrower in the longitudinal recording that is generally adopted. For making magnetic domains shorter and track width narrower, it is required to reduce distance between a magnetic head, which is for write-and-readout of information, and a magnetic recording layer. This distance is often called “spacing” in this field. The length of each magnetic domain is often called “bit length”. If spacing is wider at shorter bit length and narrower track width, write-and-readout errors would take place because magnetic flux cannot be captured sufficiently by the magnetic head.
Factor of magnetization-transition region is also important in increasing recording density. In the longitudinal recording, magnetic domains are magnetized alternatively to opposite directions along a track. Each boundary of the magnetic domains does not demonstrate clear linearity in width of the track. This is because the magnetic film is collectively made of fine crystal grains. Each boundary is formed of outlines of crystal grains. Therefore, each boundary is zigzag-shaped. Boundary of magnetic domains is called “magnetization-transition region” because it is the place where magnetization is inverted. Because each boundary is zigzag-shaped, magnetization transition averaged in track width is not sharp but gentle. This means magnetization-transition region is wider. When magnetization-transition region is wider, the number of magnetic domains capable of being provided in limited length of a track is smaller. Therefore, factor of magnetization-transition region lies as a bottleneck in enhancing recording density.
To narrow magnetization transition region, it is required to deposit a magnetic film of smaller crystal grains. For making grains smaller, making a magnetic film thinner is one solution. However, when grains are made smaller, the problem of thermal decay of magnetization becomes more serious. This point is described as follows.
When a magnetic domain is magnetized, theoretically the magnetization is sustained unless the inverse magnetic field is applied to it. Practically, however, the magnetization is dissolved slightly and slightly from the thermal decay as time passes. Therefore, permanent sustenance of the magnetization is impossible unless the magnetic domain is cooled at the absolute zero temperature. If the problem of the thermal decay appears extreme, recorded information may vanish partially after several years pass. Such a result is greatly serious in case that the magnetic recording disk is used for semipermanent information storage.
The thermal decay is a phenomenon of thermal magnetic relaxation that a magnetized particle is magnetized inversely by its thermal oscillation. Particularly, magnetized particles adjacent to a magnetization-transition region have high possibility of the thermal relaxation, i.e. inverse magnetization, from influence of the inverse field by a neighboring magnetic domain. In magnetic films for magnetic recording, such the thermal decay may take place easily when the grains are made smaller, because each grain becomes thermally unstable. Therefore, unless the problem of the thermal decay is solved, to make magnetization-transition shaper by making grains smaller may suffer difficulty.